1. Field of the Invention
The present invention relates to an image forming apparatus, in particular, to an image forming apparatus imparting gloss to an image formed on a recording sheet.
2. Description of the Related Art
As a conventional image forming apparatus, there has been proposed, for example, a system in which a second fixing device is provided on the output side of a first fixing device and in which a recording sheet having passed through the first fixing device is re-heated by the second fixing device, thereby imparting gloss to the recording sheet (See, for example, Japanese Patent Application Laid-Open No. 63-192068).
In this system, it is possible to impart gloss to the entire surface of the recording sheet. However, it is impossible to exclusively impart gloss to a limited region of the recording sheet. Further, since heat is also applied to a region requiring no gloss, such a problem as an increase in power consumption is involved.
In view of this, there is adopted a system in which a thermal head serving as a second fixing device is used on the output side of a first fixing device to re-heat an arbitrary region, whereby gloss is imparted exclusively to a region where an image is formed (See, for example, Japanese Patent Application Laid-Open No. 2007-52175, paragraph [0080]).
However, in the system discussed in Japanese Patent Application Laid-Open No. 2007-52175, before gloss has been imparted to an arbitrary region of the recording sheet by re-heating the same by using the thermal head, the water that has been contained in the recording sheet is evaporated through the heating by the first fixing device. As a result of the evaporation of the water, the recording sheet shrinks after the fixing, resulting in a change in the size thereof.
The shrinkage ratio varies according to the kind and thickness of the recording sheet. In the case of an ordinary paper sheet, the lengths thereof in the longitudinal direction and the width direction both shrink approximately 0.1 to 0.5%. For example, in the case of an A3 size ordinary paper sheet, whose size is 297 mm×420 mm, assuming that its shrinkage ratio is maximum, i.e., 0.5%, it shrinks approximately 1.5 mm in the width direction and 2.1 mm in the longitudinal direction. In the case of a resolution of 600 dpi, shrinkage, for example, by 2.1 mm corresponds to a shrinkage amount of approximately 4 pixels.
As a result, if, at the time of re-heating by the thermal head, the pattern of power supplied to each resistor of the thermal head is determined based on the image data before the fixing, the region to which gloss is to be imparted is deviated by an amount corresponding to the shrinkage of the recording sheet. And, there arises unevenness in gloss at, for example, character edge portions and image region ends.